This invention relates to a process for producing alkenyl aromatic compounds. More particularly, it relates to a process for cleaving certain diphenyl alkanes.
It is well-known that dihydroxydiphenyl alkanes can be cleaved by heating them in the presence of an alkaline catalyst to give phenol and alkenyl phenols, and several methods have been proposed for this cleavage reaction. Unfortunately, the alkenyl phenol coproduct is prone to oligomerization or polymerization under the reaction conditions. This is particularly true of p-isopropenyl phenol. Polymerization of the alkenyl phenol undesirably results in reduced yield of the desired alkenyl phenol, and requires further product purification steps to obtain the alkenyl phenol. While the dimers can be "cracked back" to the desired monomer, this requires further processing and additional expense.
An example of a prior art process is taught in U.S. Pat. No. 4,245,128, which describes a process for cleaving a dihydroxydiphenyl alkane to produce an alkenyl phenol and a polymer thereof by continuously feeding the dihydroxydiphenyl alkane into an inert organic solvent containing certain basic catalysts, heating the resulting mixture at a temperature between 150.degree. C. and 250.degree. C. at from 10 to 100 mm Hg and maintaining the concentration of the dihydroxydiphenyl alkane in the reaction medium at not more than 30 percent by weight to cleave the dihydroxydiphenyl alkane, and continuously distilling the cleavage products out of the reaction system. Said process typically produces the dimer of p-isopropenyl phenol in amounts greater than or equal to the amount of p-isopropenyl phenol produced.
Extraction using gases under supercritical conditions has been used to separate many diverse substances. For example, U.S. Pat. No. 4,247,570 and 4,251,559 describe the use of supercritical gas extractants in the decaffeination of coffee. Additionally, supercritical gas extraction has been used to remove nicotine from tobacco (U.S. Pat. No. 4,153,063), to treat hydrocarbons, including coal (U.S. Pat. No. 4,028,220) and tar sands (U.S. Pat. No. 4,108,760), and to separate isotopes of chlorine (CA 1,144,343). U.S. Pat. No. 3,969,196 discloses a method of separation which employs a supercritical gas extractant. Said method is limited to a temperature range in which the quantity of organic compound taken up by the gas phase varies inversely with the temperature. U.S. Pat. No. 4,345,976 discloses a process for separating substances of low volatility. Said process is limited in that it requires an entrainer in addition to the supercritical gas.
In view of the deficiencies of the prior art, it would be desirable to have a process for the selective preparation of p-isopropenyl phenol and related alkenyl aromatic compounds which could produce mainly the desired monomers to the virtual exclusion of dimers, polymers, etc.